Impact Block is one of the MOST important stats for ALL defense players; Clint Oldenburg (Madden gameplay lead) went on at length about this a few years ago in a blog or podcast.
A player in Match coverage uses Man Coverage rating.
However, Man and Zone are heavily "thresholded" and for most CBs and Safeties right now those are all in the 80s; All players with Man and Zone from 80 to 89 will all play more or less the same. You'll see big improvement at 90 (which you can do now with Chems or strategy items I think for that one corner for Man).
One problem with CFB is its not on PC and Mutleaks cannot look at the game code commentary to understand the (surprisingly simple) formulas and thresholds in the game.